Tropochromic coating



Patented Oct. 4, 1932 UNITED STATES PATENT OFFICE KENNETH (1 D. mcman AND LUDWIG A. STAIB, 33., OF ROCHESTER, NEW YORK, AS- SIGNORS TO EASTMAN KODAK COMPANY, OF ROCHESTER, NEW YORK, A. CORPORA- rron or new YORK TROPOCHROHIO COATING 30 Drawing.

This invention relates to tropochromic coatings and relates particularly to a new variety of such coatings.

An object of the invention is to provide 5 coated papers or other coated supports which will change their color readily, yielding a visible image, when aflected by a relatively weak stimulus.

Another object is to provide a paper which is sensitive to moderate heat, which is very stable and which is not sensitive to light.

A- iurther object is to provide a paper which is light sensitive but is not affected by heat changes'in the climatic range of temperature.

' The term tropochromic is used throughout this specification to denote any layer containing substances which have an inherent ability to change color when suitably stimu- 2o lated. Tropochromic layers are well known and in fact most hotographic reproducing materials have lig t sensitive tropochromic layers. The word tropochromic is used to denote the abilitiltlzlo change color and does not specify the d of stimulus, whether light, heat, electricity, or chemical environment, which induces the change.

In the graphic arts coatings are well known which, when applied to paper, plasticized cellulose, or other supports, can be made to change color at the will of the operator. The best known of such coatings are photo-sensitive in nature and they form the basis of the art of photography. There are,

also known, however, coatings which are thermotropic (heat sensitive) and electro tropic (sensitive to electricitg) so that color changes are recorded where eat or the electric spark are applied.

It 1s sometimes found that one and the same coating may be photo-sensitive, elcctrotropic, and thermotropic, although the maximum efiect is produced by one stimulus only. Thus, paper coated with a suitable mixture of silver nitrate and an alkali citrate darkens rapidly to light, but it will also register a color change when heat or the electric spark are applied. Light, heat, and electricity are forms of energy to which the coating is sensitive. The layer may be described as tropo- Application filed August 7,1930. Serial No. 478,784.

chromic, or possessing an inherent ability to change color, irrespective of the particular stimulus applied.

Coatings can also be affected by the application of reagents so that a local application 5 will give a localized color change or image. Thus, paper coated with an organic indicatmg dye in a slightly acid stratum turns color locally when a fine jet of ammonia is blown on to it.

Tropochromic layers may, in general, be said to contain a color generating substance or substances, so conditioned or so coupled with other substances that they yield their color change to the desired stimulus.

Of these color generating substances, silver is the best known. Coupled with an alkali bromide it can form a photographic emulsion which yields metallic silver by exposure and development; coupled with orgame acids it yields metallic silver on direct exposure to light; any layer bearing silver salts darkens to silver sulphide on the appli cation of a jet of hydrogen sulphide. I

The present invention concerns new and useful tropochromic mixtures which may be conditioned to react to various kinds of physical and chemical stimulus. The invention is more concerned with the nature of the color generating materials than it is with the particular stimulus to which the mixture can be sensitized. We have discovered that a mixture capable of registering a color change is secured whenever a metal salt capable of yielding a colored sulphide is mixed with an organic substance capable of yielding a sulphide ion. Of the many possible metals, the salts of certain elements are preferred; and

- of the many organic substances capable of yielding a sulphide ion, or available sulphur,

or an equivalent of hydrogen sulphide, certain classes of compounds are greatly preferable for the production of tropochromic layers. We have found as particularly advantageous the combination of the salt of any.

metal which yields a sulphide which is stable on exposure to the atmosphere, with any organic sulphur derivative which yields sulphide ions, hydrogen sulphide, or lts equivalent, to produce a metal sulphide, on suitable stimulus or activation.

We will now describe our inventions as applied to produce the more readily tropochromic mixtures.

For the metal salt art ofsucha mixture we use the salts of lea nickel, silver, copper, mercury, or bismuth.

For the organic sulphur derivatives we 1 use a compound where the sul hur is linked in the following way. The su phur may be joined by a double bond to a carbon atom which is associated with one or two amino groups or substituted amino groups.

Alternatively, the sulphur may be joined to a hydrogen atom (or a metal atom replacing hydrogen) by one bond and to a carbon atom by the other bond. The same carbon atom to which the sulphur is joined is also joined by a double bond to a polyvalent group or polyvalent atom other than carbon. Examples are thio-oxalic acid as /SE o and potassium thio-acetate The two forms of sulphur compounds may be tautomeric or interconvertible, or co-existent. An example is thio-acetamide, which can exist in the two forms:

JNH NH: H-S- 22 S: 40 hm hm We will now describe a specific example of a coating which is primarily heat sensitive, is somewhat less electro sensitive, and has been conditioned against light sensitiveness.

The metal salt component is a nickel salt and the organic body is thio-acetamide. A suitable coating is made thus:

Z 7 I Ewamp e I Grams Nickel acct-ate 6 -ThiO-acetamide 5 Acetic acid* 0. 5 Water 100 When this has been applied to a support, preferably paper, and dried in moderate heat it forms a slightly greenish layer which turns 68 black when touched by an object heated to less than 150 C. The unchanged salts may afterwards be washed out if deslred.

A coating which is heat sensitive and slightly light sensitive is made as follows:

Example H 10% gelatine solution 50 cc. Lead acetate 13% 20 cc. Thio-acetamide 10% 10 cc. Glacial acetic acid 1 drop A coating which utilizes the organic sulphide type of tropochromic mixture in conjunction with a separate light sensitive material is made thus:

Example HI 5% gelatine solution 20 cc.

Lead acetate 2.5 grams Thio-acetamide 0.5 to 1.0 gram Hexaminocobalticchloride 1.0 gram Acetic acid glacial 2 drops The cobalt compound liberates ammonia which causes the thio-acetamide to react with the lead acetate.)

This is an example of the invention more fully described and specifically claimed in our co-pending application Serial No. 47 3,7 85 filed August 7, 1930, for improvements in light sensitive tropochromic coatings.

Sensitive sheets coated as above may be fixed .by simply washing in water. The formulas cited are by way of example only and we consider as included within the scope of our invention any other metal coupled with any other organic sulphur providing compounds actuated primarily, or by secondary action, by an other stimulus.

examp es of metal salts which are usable, but not so convenient, we may cite the salts of antimony, arsenic, tin, molybdenum, zinc, manganese, potassium, and cobalt. Examples of sulphur bodies which are less useful are the iso-thio-cyanates, of which phenyl-iso-thio-cyanate 0 H N. C. S. is an exam la, and the di-sul hides such as cystine or dinzoyl di-sulphi e, which have doubly bonded oxygen in the molecule.

The invention is carried out by preparing a solution such as one of the examples given, and then coating a suitable support such as a good quality of paper uniformily with the mixture. After coating, the paper is dried, care being taken that the temperature during drying is not more than 100 to 125 degrees Fahrenheit. On drying, this paper is stable, and is ready to receive an impression in one of several ways. For those papers sensitive to light, exposure can of course be made in the usual way. For papers sensitive to heat or electricity, they may be exposed by contacting the surface with a stylus which is heated by electricity or other means, by a jet of hot gas, by an electric spark, or by pressing against the surface any piece of 130 recording of signals or transmitted words or pictures, an electrically heated stylus is preferable. The time of contact of the heated member and the paper is very short, practically instantaneous contact being sufficient to produce the image. As soon as the paper is subjected to heat, a visible image appears. This image may then be fixed by washing the paper briefly in warm water.

It will be seen from the above formulas that the molecular ratio of the sulphur containing body to the metal salt is quite high. This is necessary as the sulphur bearing substance must be present in sufficient quantity to strongly discolor the entire layer, that is, react with a substantial proportion of the metallic salt. To answer requirements, therefore, the substance furnishing free sulphur ions should be present in an amount of the order of that theoretically required to react with all the metal salt present, and not less than 10% of such amount.

We consider as included Within our invention all variations and equivalents coming within the scope of the appended claims.

What we claim is:

1. A tropochromic coating comprising a nickel salt and thio-acetamide.

2. A heat sensitive element comprising a support coated with a mixture containing a salt of nickel and thio-acetamide.

3. A tropochromic coating comprising a salt of thio-acetic acid and a metal salt which in the presence of free sulphur ions yields a colored sulphide, the first named salt being present in amount suilicient to furnish a quantity of sulphur ions adequate with the metal salt to Jroduce a clearly visible image consisting ot a colored sulphide.

4. A tropochromic coating comprising a metal salt which in the presence of free sulphur ions yields a colored sulphide and a salt of thio-acetic acid which will yield a sulphur ion, said salt being present in an amount not less than 10% of that theoretically required to react with the metal salt present.

5. A tropochromic coating comprising a metal salt which in the presence of free sulphur ions yields a colored sulphide and a salt of thio-acetic acid which will yield a. sulphur ion, said salt being present in an amount of the same order as that theoretically required to react completely with the metal salt present.

6. A tropochromic coating comprising a colloid carrier, thio-acetamide and a metal salt which in the presence of free sulphur ions yields a colored sulphide, the thio-acetamide being present in an amount of the same order as that theoretically required to react with substantially all of the metal salt present.

7. A heat sensitive element comprising a support coated with a carrier layer containing a salt of thio-acetic acid and a metal salt which in the presence of free sulphur ions yields a sulphide of a color contrasting with the color of the support, the organic salt being present in an amount of the same order as that required to react with substantially all of the metal salt present.

8. A tropochromic coating comprising a lead salt and thio-acetamide.

Signed at Rochester, New York, this 2nd day of August 1930.

KENNETH C. D. HICKMAN. LUDWIG A. STAIB, JR.

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